The present invention relates to a technical field of a control system of an engine in which a purge gas containing evaporated fuel desorbed from a canister is supplied to an intake passage.
Conventionally, arts are known in which when it is determined that evaporated fuel easily overflows from a canister during a deceleration fuel cutoff of an engine, a purge gas containing the evaporated fuel desorbed from the canister is supplied to an intake passage of the engine. For example, JP2007-198210A discloses such an art. By supplying the purge gas to the intake passage during the deceleration fuel cutoff as above, the overflow of the evaporated fuel from the canister can be suppressed. Although the evaporated fuel within the purge gas supplied to the intake passage will be discharged unburned to an exhaust passage through the engine, the unburned evaporated fuel can be purified by an exhaust emission control catalyst provided in the exhaust passage.
Further, in JP2007-198210A, a linear O2 sensor for detecting an oxygen concentration within exhaust gas for the purpose of performing a feedback control of an air-fuel ratio within a combustion chamber is provided upstream of the exhaust emission control catalyst, and an O2 sensor is provided downstream of the exhaust emission control catalyst.
Meanwhile, the O2 sensor located downstream of the exhaust emission control catalyst is normally for detecting whether a state of the air-fuel ratio of the exhaust gas is stoichiometric, rich, or lean. When the air-fuel ratio is stoichiometric or rich, an output value (output voltage) of the O2 sensor indicates a first voltage (e.g., approximately 1V), and when the air-fuel ratio is lean, the output value indicates a second voltage (e.g., approximately 0V) which is lower than the first voltage. Therefore, in a situation where the exhaust gas passing through the O2 sensor is assumed to be rich immediately before the deceleration fuel cutoff, when the deceleration fuel cutoff is performed in this situation, due to the deceleration fuel cutoff, the output value of the O2 sensor changes from the first voltage to the second voltage in a short period of time in an early stage of the deceleration fuel cutoff.
Here, there is a case where an abnormality occurs in the O2 sensor and a speed of a change of the output value of the O2 sensor (a speed of the change from the first voltage to the second voltage) caused by the deceleration fuel cutoff becomes lower or the output value does not reduce to the second voltage. Therefore, determining whether the O2 sensor is abnormal (performing an abnormality determination), based on the change of the output value of the O2 sensor caused by the deceleration fuel cutoff, may be considered. For example, when the speed of the change of the output value of the O2 sensor caused by the deceleration fuel cutoff (e.g., the changing speed between the first (high) and second (low) voltages (i.e., a changing period of time between the two voltages) is lower than a predetermined speed (longer than a predetermined period of time), the O2 sensor is determined to be abnormal.
However, by supplying the purge gas to the intake passage of the engine during the deceleration fuel cutoff (performing a purge) as JP2007-198210A does, the purge is performed also during the abnormality determination, and due to the existence of the evaporated fuel within the purge gas, the speed of the change of the output value of the O2 sensor caused by the deceleration fuel cutoff becomes lower. As a result, even if the O2 sensor is normal, it may be falsely determined as abnormal.